Store Cygwin on an ISO image for easy re-installation onto virtual machines.
I'm installing Cygwin onto a Parallels virtual machine. I wanted to download Cygwin and its packages only once, and to install it onto multiple virtual machines. I tried storing it as a directory on the Mac, and attaching it to the Parallels virtual machine as a shared folder. Unfortunately, shared folders appear as a network drive on ".psf" under Parallels, and Cygwin has problems installing from it. Of course, I could have copied the files onto the (virtual) C: drive and installed it from there, but would have needlessly used up space on the VM's drive.
The solution I found was to create an ISO disc image containing the Cygwin files and to mount that onto the virtual machine as a DVD disc. Cygwin installs fine from the virtual DVD-ROM and unnecessary file copying was avoided.
Creating the ISO still required the packages to be downloaded inside a VM running Windows, and then copied out of that VM into an ISO image. But after that, no more copying is required.